Method for producing a panel with a wood based core and a surface layer comprising a thermosetting resin

ABSTRACT

A method for forming of a building panel with a surface including a thermosetting resin such that tension created during curing of the surface is reduced or eliminated.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. provisionalapplication no. 61/532,753, filed on Sep. 9, 2011.

TECHNICAL FIELD

The disclosure generally relates to the field of forming of panels,e.g., building panels. More particular, the disclosure relates to amethod of forming floor panels and floor panels produced by the method.

KNOWN TECHNOLOGY

Traditional laminated panels, intended to be used for, e.g., flooring orfurniture components, are produced by the following steps:

-   -   applying a melamine formaldehyde resin impregnated kraft paper        as balancing layer on one side of a core of wood fibre based        material for example HDF;    -   applying a by melamine formaldehyde resin impregnated printed        decor paper on the other side of the core;    -   applying a melamine formaldehyde resin impregnated transparent        overlay paper with wear resistant particles, for example        aluminium oxide, on the décor paper; and    -   curing the resin by applying heat and pressure in a continuous        or discontinuous press to obtain a laminated product.

Typical press parameters are 40 bar pressure and a temperature of160-200° C. with a pressing time of 12-30 seconds. The surface layer hasgenerally a thickness of 0.1-0.2 mm, the thickness of the HDF corevaries between 6-12 mm and the balancing layer is about 0.1-0.2 mmthick.

This production method and product produced by such methods aregenerally referred to as the DPL process and DPL products (DirectPressure Laminate)

Recently new panels with a wood powder based surface have beendeveloped. Powder comprising wood fibres, binders preferably melamineparticles, aluminium oxide particles and color pigments are scattered ona HDF board and pressed under heat and pressure in a continuous ordiscontinuous press to obtain a product with a paper free and solidsurface layer. The press parameters are similar to DPL. A higherpressure of 40-80 bar and a longer pressing time of 20-40 seconds may beused when the surface is formed with deep embossing and a thickness offor example 0.4-0.6 mm. The pressing temperature is generally 160-200°C. Such wood fibre based floors, generally referred to as WFF floors,have considerably better properties than traditional laminate floorssince a ticker and more impact-resistant and wear-resistant surface maybe produced in a cost efficient way.

FIGS. 1a-1d show production of a laminate or WFF floor panel accordingto known technology.

A surface layer 2 of an impregnated overlay and a decorative paper or apowder layer is applied on the upper part of a HDF core 3. A balancingpaper or powder-backing layer 4 is applied on the backside of the HDFcore as shown in FIG. 1a . The core 3 with the upper 2 and lower 4layers is moved into a press and pressed under heat and pressure suchthat the layers are cured and attached to the board as shown in FIG. 1b.

The layers on the front face and the rear face are exposed to a firstshrinking when the thermosetting resin in the upper and lower layercures during pressing. The backing layer at the rear face balances thetension that is created by the surface layer of the front face and thepanel is substantially flat with a small convex backward bending when itleaves the press. Such first shrinking and balancing of the panel isbelow referred to as “pressing balancing”. The second temperatureshrinking, when the panels is cooled from about 160-200° C. to roomtemperature, is also balanced by the backing layer and the panel 1 isessentially flat as shown in FIG. 1b . The second balancing is belowreferred to as “cooling balancing”. A small convex backward bending ispreferred (not shown) since this counteracts upward bending of the edgesin dry conditions when the relative humidity may go down to 20% or lowerduring wintertime.

The problem is that this essentially flat panel comprises tension forcescaused by the shrinking of the surface and balancing layers.

The panel is generally cut into several elements and formed into floorpanels with locking systems on long and short edges as shown in FIG. 1c. The locking system may comprise, for example, generally a tongue 10and a tongue groove 9 for vertical locking and a strip 6 with a lockingelement 8 that cooperates with a locking groove 14 for horizontallocking.

The surface and the core will swell in summertime when the indoorhumidity is high and shrink in wintertime when the indoor humidity islow. The panels will shrink and expand and a cupping of the edges maytake place. The balancing layer is used to counteract such cupping. Thisbalancing is referred to as “climate balancing”.

FIG. 1d show that the internal tension may cause problems when thelocking system is formed and especially when the panel is installed in adry climate, which causes the layers to shrink. A so called topping ofthe edges may occur when the internal tension opens up the groove 9 andbends the tongue part 10 upwards due to the fact that these parts are nolonger balanced when the locking system is formed. The internal tensionmay also cause a backward bending of the strip 6, which decreases thelocking strength and the quality of the locking system. The tension maybe higher than the internal bonding strength of the core, which maycause cracks C, primarily in horizontally extending grooves, such as thetongue groove 9. Such internal tension may require a higher boardquality than normal and this may increase the costs.

It would be a major advantage if the tension caused by the curing andtemperature shrinking could be reduced or completely eliminated. Such atension reduction would be especially favourable in WFF floors thatgenerally comprise rather thick solid surface layers with considerableamount of thermosetting binders or high quality DPL floors forcommercial application where the thickness and the resin content of thesurface layer is higher than in DPL floors for domestic use.

SUMMARY OF THE DISCLOSURE AND OBJECTS THEREOF

An overall objective of embodiments of the present disclosure is toprovide a method to reduce or eliminate tension in panels with a surfacecomprising thermosetting resins that is cured and connected to a coreunder heat and pressure. Embodiments of the present disclosure maycombine various process flexibility and product features as listedbelow.

A first aspect of the disclosure is a method for producing a panel, forexample a building panel, preferably a floor panel, with a wood basedcore and a surface layer comprising a thermosetting resin wherein themethod comprising the steps of:

-   -   curing and connecting the surface layer to the core under heat        and pressure in a first main pressing step;    -   applying a bending force on the panel after the first main        pressing step such that it bends backwards away from the surface        layer in hot condition; and    -   releasing the bending force such that the panel springs back in        an essentially flat shape.

In an embodiment, the surface layer may be directly connected to thecore. In an embodiment, the surface layer may be connected to anintermediate layer, the intermediate layer being directly connected tothe core.

The method may further comprise the step of decreasing the surfacetemperature during bending.

The temperature decrease may be about 20° C.

The temperature decrease may be more than 20° C.

The resin may be a melamine formaldehyde resin.

The method may comprise a bending that is at least about 3 cm/m.

The surface layer may comprise wood fibres and wear resistant particles.

The surface layer may comprise wood fibres, wear resistant particles andcolor pigments.

A second aspect of the disclosure is a floor panel comprising a woodbased core and a surface layer comprising a thermosetting resin producedaccording to the first aspect and that the panel has no balancing layer.

The floor panel according to the second aspect may have a back face,which is the wood based core.

The floor panel may be a DPL panel comprising a surface layer ofimpregnated paper bonded to a HDF core.

The floor panel may be a WFF panel comprising a powder based surfacelayer.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will in the following be described in connection toexemplary embodiments and in greater detail with reference to theappended exemplary drawings, wherein:

FIGS. 1a-1d show known production of a floor panel according to knowntechnology.

FIGS. 2a-2d show production of a panel according to a first embodimentof the disclosure.

FIGS. 3a-3d show production of a panel according to a second embodimentof the disclosure.

FIGS. 4a-4c show a method to measure internal tension in a panel.

FIGS. 5a-5b show the panel temperature during pressing and cooling.

FIGS. 6a-6d show a method to obtain plastic deformation in a panel.

DESCRIPTION OF EMBODIMENTS OF THE DISCLOSURE

FIG. 2a shows a panel 1 that has been pressed with only a surface layer2 applied on a core 3. The panel may be a DPL panel or a WFF panel. Theshrinking of the surface layer 2 during pressing, when the thermosettingresin has cured, causes the panel edges 1 to bend upwards. The bendingmay increase further when the panel 1 is cooled down to roomtemperature. The bending is such that the panel 1 is not possible to usefor flooring production. The reason is that there is no backing layerthat balances the shrinking of the surface layer 2 during pressing andcooling.

The panel has essentially the same surface temperature as the presstable when it leaves the press. A normal pressing temperature is about170° C. The core has generally a much lower temperature of about 80-100°C. This means that the surface layer will cool down very quickly afterthe pressing. After 10 seconds, the surface temperature may be 120° C.and after 15-20 seconds only about 80° C. The cooling of the surface andthe core is effected by the pressing time. A longer pressing time willincrease the core temperature more than a short pressing time.

FIG. 2b shows that the panel 1 is pressed in a second pressing stepbetween curved pressing tables 5, 7 when the surface layer 2 is stillhot after the first pressing step. If such post pressing is made at atemperature of about 70-90° C., it will deform the hot HDF core andcreate a counter pressure that is sufficient to produce a flat panelwithout a backing layer. A plastic non-reversible deformation will takeplace.

A post pressing at higher surface temperatures will also stretch anddeform the surface layer 2 and eliminate the tension caused by theshrinking in the first main pressing step. Such surface stretching ispreferably made at temperatures around 140-160° C.

The second pressing and surface stretching step is preferably a coldpressing and it is an advantage if the panel is moved to the secondpressing and bending station as soon as possible in order to maintain ahigh initial surface temperature when the bending starts. For example,the peak surface temperature may be in the range of 160-200° C., forexample at about 170° C. during the first pressing, where the surfacelayer is connected to the core. When the bending step starts the surfacetemperature may be at least at 140° C., or at least 120° C. For example,when the bending may step starts the surface temperature may be within30° C., or within 40° C., or within 50° C., of the peak surfacetemperature during the first pressing, where the surface layer isconnected to the core. A decrease of the temperature in bended andstretched position will generally be obtained automatically when thecore cools down the surface. This will stabilize the surface layer in astretched position and a permanent deformation may be obtained. Internaltension in the panel may be reduced considerably or completelyeliminated and the panel may be completely flat when it is cooled toroom temperature as shown in FIG. 2 c.

The production balancing and the cooling balancing that is made with abacking layer may be replaced partly or completely with plasticdeformation.

The bending may be such that the panel is convex and bends backwardswhen it leaves the second pressing station. The panel may be slightlybended in a third strep upwards towards the surface such that apre-determined shape is obtained. This type of overstretching and doublebending may reduce the internal tension completely. The temperatures,bending time and bending magnitude should be adapted to the quality andspecification of the surface and the core.

Heat may be added in the second surface-stretching step in order toimprove and control the stretching and to adapt the production method tovarious surface thickness and resins. The surface may also be cooled inorder to stabilize the panel in a pre-determined condition.

The second post pressing operation and the bending of the panel may bemade in several ways. It is preferred that the bending and stretching isthree-dimensional such that the bending is made along and across thepanel. The press tables are preferably curved in two perpendiculardirections like a top of a ball.

The backward bending may also be made with pressing cylinders, frames,pressing against a rubber or silicone matrix, with rollers and in manyother ways where the panel is bended in hot condition after the initialmain pressing operation where the thermosetting resin is cured.

The thermosetting resin may be adapted such that it is easy to stretch.Resins similar to the types used in so-called post-forming laminatequalities may facilitate surface stretching by bending.

The surface layer may also be stretched by lateral pulling forcesapplied at the panel edges.

It is also possible to heat up the panel again after cooling in aseparate production step with for example a hot press table, infra-redlamps and similar in order to soften and stretch the surface layer inorder to obtain a flat panel. A first cylinder may bend the panelbackwards along the length and a second cylinder may, after preferablyan intermediate heating, bend the panel along the width. The panels mayalso be held in a bended position in the cooling part after thepressing.

FIG. 2d shows that a moisture barrier 4 may be used of for example aplastic foil, treated paper, foam etc. in order to prevent moisture topenetrate into the panel from the sub floor. Such a backing layer may beused to obtain the climate balancing. In some application the bending ofthe HDF core may be sufficient even for the climate balancing.

The Surface Stretching Process (SSP) according to embodiments of thedisclosure may offer one or more or all of the following advantages:

-   -   a more cost efficient board with lower internal bonding strength        may be used;    -   topping that causes high wear at the edges and decreases locking        strength due to internal tension may be eliminated;    -   splitting of the board in the locking system may be avoided; and    -   cost savings may be obtained by eliminating the backing or        replacing the paper/melamine backing with a thinner or more cost        efficient material.

FIG. 3a shows an embodiment with a thin backing layer 4 that is not ableto balance the panel 1 after initial pressing. FIG. 3b shows bending andpreferably also stretching of the surface layer 2. The result is a flatpanel as shown in FIG. 3c with a backing layer 4 and reduced internaltension. The backing layer prevents moisture to penetrate into thebackside of the panel 1 and will stabilize the panel when humiditychanges. FIG. 3d shows three-dimensional bending of a panel 1. Themagnitude of the bending B is preferably about 3-6 cm/m or more.

FIGS. 4a-4c show a method to measure internal tension in a panel. Agroove 11 is formed in the panel edge. A high internal tension willseparate the edges as shown in FIG. 4b . A large difference between theinner panel thickness T1 and the outer panel thickness T2 means that thepanel 1 has a large internal tension that may cause topping and cracksin the locking system, especially in dry conditions. FIG. 4c shows apanel 1 with a low internal tension. For example, a panel with a reducedinternal tension will have a T2 that is lower than a T2 for the samepanel that did not have method, such as the bending step, performed onthe panel.

This measuring method may be used to adjust the parameters of thebackwards bending in the second pressing step by modifying initialpressing temperature, final pressing temperature, pressing time and themagnitude of the bending.

FIG. 5a shows the temperature curve during pressing of a WFF floor panelwith a 0.5 mm surface layer. The temperature A is measured in ° C. andthe time B in minutes. The upper curve C is the temperature of thesurface layer and the lower curve D is the temperature of the coremeasured 3 mm below the surface. The surface temperature is increasingduring pressing to 170° C. and the core temperature to about 110° C. Thecooling of the surface layer takes place rather quickly as may be seenfrom FIG. 5b that shows the same temperature curves C and D during thefirst 60 seconds. For example, the bending step may start within 60seconds or less, or 90 second or less, from when the first pressingstarts. For example, the bending step may start within 30 seconds orless, from when the first pressing step ends. The plastic deformation ispreferably made when the surface temperature is at least about 140° C.and when the core temperature, measured a few mm below the surface, isat least about 100° C. It is possible to modify the resins in the coreand in the surface layer such that plastic deformation may be made atlower temperatures for example at 120-140° C. for the surface layer and80-100° C. for the core.

FIGS. 6a-6d show that vacuum cups 15 with adjustable cylinders 16 may beused to bend the pressed panel 1 after pressing. The vacuum cups maylift the panel immediately after pressing and form it to apre-determined shape as shown in FIGS. 6b and 6c . Such forming may bemade within for example 10-20 seconds after the pressing when the panelsurface still has a temperature around and above 140° C. The panel iskept in a bended position until it cools down to a suitable temperature,preferably below 100° C., and a permanent non reversible plasticdeformation is obtained such that the panel is essentially flat at roomtemperature as shown in FIG. 6 d.

Prophetic Example: Second Pressing And Stretching Step:

Panel size 1200 * 2400 * 9 mm Surface 500 gr. powder mix/m2 Initialpressing temperature 140-160° C. Second pressing temperature 80-140° C.Pressing time 5-15 sec Bending - width 5 cm Bending - length 14 cm

The invention claimed is:
 1. Method for producing a panel with a woodbased core and a surface layer comprising a thermosetting resin, whereinthe method comprises the steps of: curing and connecting the surfacelayer to the core while applying heat and pressure in a first mainpressing step, thus raising a temperature of the surface layer above aninitial temperature; while the panel is still above the initialtemperature from the first pressing step, applying a bending force onthe panel in a second pressing step to bend the panel such that anuppermost surface of the panel is convex and a lowermost surface of thepanel is concave; and releasing the bending force such that the panelsprings back to an essentially flat shape.
 2. The method as claimed inclaim 1, wherein the method further comprises the step of decreasing thesurface temperature while applying the bending force.
 3. The method asclaimed in claim 2, wherein the surface temperature decrease is about20° C.
 4. The method as claimed in claim 2, wherein the surfacetemperature decrease is more than 20° C.
 5. The method as claimed inclaim 1, wherein the thermosetting resin is a melamine formaldehyderesin.
 6. The method as claimed in claim 1, wherein the bending is atleast about 3 cm/m.
 7. The method as claimed in claim 1, wherein thesurface layer comprises wood fibres and wear resistant particles.
 8. Themethod as claimed in claim 1, wherein the surface layer comprisesimpregnated paper, the wood based core comprises a high-densityfibreboard (HDF) core, and the impregnated paper is bonded to the HDFcore.
 9. The method as claimed in claim 1, wherein the surface layercomprises wood fibres, wear resistant particles and colour pigments. 10.The method as claimed in claim 1, wherein the surface layer comprises apowder based layer, the powder comprising wood fibers, wear resistantparticle and colour pigments.
 11. The method as claimed in claim 1,wherein the panel is a building panel.
 12. The method as claimed inclaim 1, wherein the panel is a floor panel.
 13. The method as claimedin claim 1, wherein the bending force is applied to at least twoopposite edge portions of the panel.
 14. The method as claimed in claim1, wherein the panel is rectangular and the bending force is applied toboth sets of opposite edge portions of the panel.
 15. The method asclaimed in claim 1, wherein the bending force is applied with a firstpress table on the uppermost surface of the panel and a second presstable on the lowermost surface of the panel.
 16. The method as claimedin claim 15, wherein the first and second press tables are curved in twoperpendicular directions, such that bending is made along and across thepanel.